The elements are shown in Fig. 67, lead grid _L_, which is filled in with paste of peroxide of lead, and which neither buckles nor disintegrates. The zinc _Z_, however, possesses a novel feature. A cavity is cast in the zinc element and filled with an amalgam of mercury, the copper electrode passing through this amalgam into the solid zinc, as shown in the cut. As the action of the battery proceeds, this amalgam forces its way into the pores of the element and keeps up so good an amalgamation of both copper rod and zinc that zincs can be used up to a point when the rising internal resistance makes it economy to throw them away, and absolutely no perceptible local action takes place in the cell upon continued open circuit. A preparation is furnished if desired, which forms a jelly of the electrolyte, making the cell readily portable. Like all of these combinations, its electromotive force exceeds two volts, and its internal resistance is low enough to advise its employment in coil work.
When a storage battery is to remain unused for a long time it must first be fully charged, and then every week or so the charging current passed through it until it bubbles. Where it is to be laid away for a long period of time, and weekly charging is not feasible, the following operations are necessary: First, fully charge battery, remove electrolyte, and replace by water immediately. Discharge at normal rate until voltage runs down to 1.7 per cell. Gradually decrease resistance until battery is almost on short circuit. Let it stand for a day, then pour off the water, and keep elements in a dry, clean place.
The currents of high frequency used by Tesla in his researches are produced by electrical rather than mechanical means. The alternating current dynamo used by him renders a current of 10,000 alternations per second, but the actual current necessary to the performance of the luminous effects has a frequency of millions of oscillations per second, produced by the discharge of Leyden jars or condensers.
Dr. Oliver J. Lodge, in his "Modern Views of Electricity," shows that the discharge of the Leyden jar is in general oscillatory, the apparently single and momentary spark, when analyzed in a very rapidly rotating mirror, is shown to consist of a series of alternating flashes, rapidly succeeding one another and lasting individually less than one hundred thousandth of a second. The capacity of the condenser and inertia of the circuit regulate the rapidity of these oscillations. A 1 microfarad condenser discharging through a coil of large self-induction, such as one having an iron core, may oscillate only a few hundred times per second. On the other hand, a Leyden jar of the 1 pint size discharging through a short circuit will set up oscillations, perhaps ten million per second; and a still smaller jar would give oscillations away up in the billions. But these small jars are quickly discharged, and require a constant replenishing.